text: The direct reduction of iron ore principle was tested in the late 19th century using high-temperature stirring of ore powder mixed with coal and a small amount of limestone to adjust the ores acidity. Carl Wilhelm Siemens direct reduction process, which was sporadically employed in the United States and United Kingdom in the 1880s, is particularly noteworthy. This process is based on using a 3-meter in diameter and similarly lengthy drum with a horizontal axis for blowing gases preheated by two regenerators.
  The metallurgy industry underwent much research regarding the implementation of rotary tubular furnaces, inspired by similar equipment used in cement works. The Basset process, developed during the 1930s, is capable of even producing molten cast iron. In the 1920s, German metallurgist , head of the metallurgy department at the  and professor at the Clausthal University of Technology, explored the metallurgical applications of this type of furnace. He filed a series of patents for removing volatile metals from steel raw materials.
  During the 1930s Johannsen initiated the development of direct-reduction iron production. The first installation underwent testing from 1931 to 1933 at the Gruson plant in Magdeburg. Research on the Krupp-Renn process continued until 1939 at the Krupp facility in Essen-Borbeck. The process, named after the Krupp company that created it and the , translating to "low furnace," displayed potential. As a result, Krupp procured patents overseas to safeguard the invention after 1932.